[Cocoa] Refactor: remote MediaPlayerPrivateAVFoundation::scheduleMainThreadNotification()
<rdar://problem/118335975>
Pull request: https://github.com/WebKit/WebKit/pull/20419
Committed 270659@main (58e56ea67c7b): <https://commits.webkit.org/270659@main> Reviewed commits have been landed. Closing PR #20419 and removing active labels.